The present invention relates to a filter device including sheets having predetermined optical characteristics and, more particularly, to a filter device mounted in a backlight device for illuminating a transmission type liquid crystal display panel from the rear surface side.
A filter device including sheets having predetermined optical characteristics has been used. For example, this filter device is applied to a backlight device for illuminating a transmission type liquid crystal display panel from the rear surface side. The filter device applied to such a backlight device includes a diffusion sheet for diffusing light from a light source, a prism sheet for increasing the luminance, and the like.
To clamp and support these sheets, this filter device also includes a frame member having a frame portion formed in correspondence with the peripheral portion of a sheet and a side cover constituted by press members attached to the respective sides of the frame portion. The sheet is attached to the frame member such that two opposing sides of the sheet are clamped between the frame portion and the side cover.
In the filter device designed to press two opposing sides of a sheet against the frame portion of the frame member by using the side cover as described above, the sheet must be accurately positioned with respect to the frame portion. In addition, the sheet must be kept stretched. If these requirements are not satisfied, the sheet crinkles, resulting in a deterioration in optical characteristics. In addition, when a plurality of sheets are stacked and attached to the frame member, since the sheets must be visually positioned with high precision, it takes much time and effort to accurately position the respective sheets. It is difficult to keep the respective sheet stretched. Furthermore, when a sheet needs to be replaced, a cumbersome operation which may damage the sheet is required. As a result, the working load on the operator may increase. In addition, it is difficult to reduce the number of parts and the manufacturing cost.